Granular gases are granular materials where the duration of a
collision is much shorter than the typical collision time
[1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}.
This situation can be obtained by either placing a dilute particle
system in a micro-gravitational environment (e.g. during a parabolic
flight [9]}), or experimentally easier, by feeding the
system with energy such that a gaseous steady state appears (e.g. by
vertically vibrating the enclosure [10]}, [11]}). For a granular
gas, in the dilute limit, binary collisions dominate over multiple
collisions. Contrary to molecular gases, granular gases are
dissipative. So the continuous loss of kinetic energy due to collisions
not only makes the gas cool down but is also accompanied by collective
phenomena such as cluster formation or shear banding. Granular gases
are subject to instabilities and cluster formation
[12]}, [3]}, [10]}, [6]}, [16]}, [17]}, [18]},
deviations from the Maxwell-Boltzmann velocity distribution
[19]}, [20]}, phase transitions [21]}
and the formation of vortices [3]}.
